Glass research in the past 25 years has been focused on developing glasses from different and often more exotic materials, and on new methods of formation such as the sol-gel process, in order to extend their use. For example, the emphasis in fiberoptics has been on the purity of silica glass to enable greater signal transmission which is diminished by the light absorption of minor impurities. The emphasis in laser glasses has been to explore new compositions which extend the wavelengths for laser transmission and which will not shatter when exposed to the intensity of the laser beam.
The field of graded glass--glass whose optical, chemical and physical characteristics vary throughout the material in contrast to cladded glasses whose properties change abruptly at their interfaces--is a new science. Considerable effort has been expended by glass scientists to produce glasses with graded properties using techniques such as ion exchange, sol-gel preparation, vapor deposition, and ion implantation but these have been successful only for very thin glasses at best.
For example, the practical limitations for the various prior art processes are as follows:
______________________________________ Process Maximum Size .DELTA.n ______________________________________ CVD 0.1 mm 0.01 Ion exchange 10 mm 0.04 Ion stuffing 50 mm 0.04 Sol-gel 30 mm 0.1 ______________________________________
Probably the only significant commercial breakthrough in graded glass has been graded fiberoptics. Because fibers are so thin and the depth of grading (change of refractive index) is so slight, some of the available processes are applicable. However, those processes currently available cannot be used in any large geometry lenses (i.e., camera lenses) or any micro lenses (i.e., compact disk lenses) that require greater or more sophisticated grading.
Glass experts have been convinced that the problems inherent in fabricating graded glass in any significant size or degree of gradation were formidable. The technical barriers were perceived as so great that the scope of research has been limited.
Recently, however, a fabrication process has been developed for producing graded glass which is unfettered by the limitations of earlier processes. The fabrication process, disclosed in U.S. Pat. Nos. 4,883,522 and 4,907,864 and application Ser. No. 07/266,670, now U.S. Pat. No. 4,929,065 involves fusing glasses of varying compositions together to achieve a glass body having a graded index of refraction. The two patents deal with forming bi-directional gradients, starting from frits of different compositions, while the patent application deals with a process for fusing glass plates of different compositions together.
The advantage of graded glass over conventional glass is straight-forward. Since conventional glass is essentially homogeneous, that is, having basically the same characteristics throughout, any optical device requiring a variation of characteristics to solve an optical problem requires a complex system comprised of many glass elements.
Since graded glass can incorporate a mix of many characteristics into one piece of glass, the number of elements can be reduced significantly. Fewer elements means a significant reduction in materials, labor, size of lens housing, and weight. The result is a cheaper, lighter optical device with the same or improved optical characteristics.
The virtues of graded glass not only can improve conventional optical systems, but can improve laser technology and a whole range of applications relating to fiber optic telecommunications, defense, and avionics. Graded glass can also make many new devices commercially viable--such as photonic circuits and solar energy devices.
Earlier processes developed to produce graded glasses are limited both in the characteristics which may be varied and the thickness of the product. The recently-developed fabrication techniques, embodied in the patents and the patent application listed above, allow the grading of numerous characteristics over a wide range of glass sizes. This new technology is expected to spawn a whole new generation of glass products and glass applications.
As this technology develops, new methods are required to enable those skilled in the art to fabricate glass bodies having a desired refractive index graded profile, with certain other properties, such as thermal coefficient of expansion, glass fusion temperature, Abbe value, density, and others, predetermined by choice.